NAME

gprep -- transform shifts for one WFPC2 chip into parameters for mosaicing all four chips

USAGE

gprep image axsh aysh arot

PARAMETERS

image [image file name]

The name of the input image file. Normally this comes from the output of avshift. This image name is just transferred to the output -- the image isn't accessed.

axsh = 0.0

The X pixel shift from avshift. This is the shift which avshift derives in order to register the reference chip (chip 3).

aysh = 0.0

The Y pixel shift from avshift. This is the shift which avshift derives in order to register the reference chip (chip 3).

arot = 0.0

The rotation angle from avshift. This is the rotation which avshift derives in order to register the reference chip (chip 3).

(xsh = 0.0)

An additional shift to be applied in X to the mosaic after the avshift ones have been applied. The units are output pixels.

(ysh = 0.0)

An additional shift to be applied in Y to the mosaic after the avshift one has been applied. The units are output pixels.

(rot = 0.0)

An additional rotation (degrees anticlockwise) to be applied to the mosaic after the avshift one has been applied.

(scale = 1.0)

An additional scale factor to be applied to the mosaic after the avshift one has been applied.

(wfpc2_chips) [pset]

Pset containing the WFPC2 chip offset parameters.

DESCRIPTION

This task takes the output values from avshift and outputs the drizzle parameters which can be used to mosaic all four WFPC2 chips. The avshift values which are used as input are for the reference chip (WF3). The resultant mosaic may be shifted, scaled and rotated as desired using the hidden parameters (xsh, ysh, rot and scale).

The reference chip is WF3 so the avshift results appropriate to it should be used as input.

The output parameters are those needed by drizzle and must be applied with shft_fr=output and shft_un=output.

Note that the default xsh=ysh=0 is not normally optimal as it will put WF3 central in the output rather than the center of the overall mosaic.

This script contains hard-coded values for the relative positions and orientations of the four chips. Although the angles and scales don't change significantly with time the shifts do - by amounts comparable to one WFC pixel (0".1). The numbers in this version were obtained at the time of the HDF-S observations in late 1998 and may well not be appropriate at other epochs.

EXAMPLE

Let's say that we have a set of images which have been processed using the dither package up to the avshift stage. For one of them the output from avshift looks as follows:

# Image Group xsh_in  ysh_in best_xsh  best_ysh tot_sh_in del_xsh  del_ysh  rot
N1.hhh  1  117.264  193.413  130.043    9.062  130.359  -12.779   184.351  1.280
N1.hhh  2   -7.082  -55.543   -7.209  -55.502   55.968    0.127    -0.042  1.280
N1.hhh  3  -71.224    7.152  -71.224    7.204   71.587   -0.000    -0.051  1.280
N1.hhh  4   -8.079   71.050   -8.282   71.095   71.576    0.203    -0.045  1.280

We now want to mosaic this image, all four chips, so that it lines up with
its related dithered images and with the output shifted by 800,800 and a 
scale of 0.5. The parameters of 'gprep' could then be as follows - note that
the input values are those of the reference chip - WF3:

image   =               N1.hhh  Input image file name from avshift
axsh    =              -71.224  Best chip 3 X shift from avshift
aysh    =                7.204  Best chip 3 Y shift from avshift
arot    =                 1.28  Rotation from avshift
(xsh    =                 800.) X shift for reference chip 3
(ysh    =                 800.) Y shift for reference chip 3
(rot    =                   0.) Rotation angle (degrees anticlockwise)
(scale  =                  0.5) Scale factor

The result will be:

Image Group  xsh      ysh       rot       scale
N1.hhh 1  -378.826  -235.996  -178.510   1.0926
N1.hhh 2   699.143  -598.098   -89.030   0.4998
N1.hhh 3   657.266   811.222     1.280   0.5000
N1.hhh 4  -788.485   777.480    91.830   0.4996

These parameters can then be fed to 'drizzle' (with 'shft_fr=output' and
'shft_un=output') to produce the mosaic.

BUGS

The chip-to-chip mappings are hard-coded. The reference chip is forced to be WF3.

REFERENCES

This task was written by Richard Hook and Andrew Fruchter. It was incorporated into the dither package by J.C. Hsu.

SET ALSO

avshift, drizzle, offsets, shiftfind